This invention relates to a new and useful device for testing stationary valve regulated and vented lead acid batteries, which provides a prediction of imminent battery failure by means of monitoring a slight change in voltage across the cell or battery terminals, while it is in "float" service, that is neither charging or discharging under use.
The Applicant's device includes an electronic instrument for battery testing, having a single unit electronic instrument capable of detecting slight changes in voltages which affect battery performance. Such variations in voltage are functions of either a short or an open situation, wherein the internal resistance of the battery changes. As a result, the cell voltage is altered across the battery in relation to the voltage drop across the battery.
The device will note such a voltage change while the battery is in a float situation for a pre-set period of time. If the voltage change continues for the duration of a pre-set period of time, such as twenty four hours, the device will indicate possible failure mode by means of both audible and visual alarms. The device may also provide a C form contact for remote indication, and a single digital output.
Applicants' device is not an indicator of a specific voltage, but rather it sets up a benchmark window to see when one battery or cell in a plurality of cells moves up or down in voltage out of the prescribed range of acceptable voltage. The device takes advantage of a predicted rate of failure to alert the user of imminent failure.
Applicant's device is necessary, because batteries are subject to changes in resistance caused by short or open situations, which will eventually result in battery failure.
Therefore the invention will predict with some certainty the appearance of an open or short situation that would invariably result in a battery failure. Since this technique is not currently in use commercially the advantage of this invention over other more laborious and costly methods of predicting failure would then be realized.
The condition of a battery depends upon chemical reactions within the battery. Chemical reactions within lead based batteries are enhanced or decreased depending upon the condition of the plates. If active material sheds, it will be caught in the space between the plates, resulting in a short or less impedance. The shedding process can cause a mixture of shedded plate material with the sulphuric acid medium between the plates. When this action occurs a process similar to plating, involving electrolysis of the suspended active material will create a coating or plating action on the outer walls of the plates within a battery. This creates a layer of lead sulfate that adheres permanently to the surface of the plates and ultimately retards the batteries ability to deliver adequate current. This process is known in the industry as sulphation. Furthermore, normal expansion or contraction of the battery plates or contacts may cause an open situation. This is a result of normal battery use.
Typical battery performance problems, such as grid corrosion, contamination, improper temperature, overcharging and undercharging prematurely age a battery, permanently reducing its functional capacity. Such performance problems are discussed in the specification pamphlet entitled "The Dynasty Battery" of Johnson Controls, Inc., Specialty Battery Division, 900 East Keefe Avenue, Milwaukee, Wis. 53201. The Johnson Controls, Inc.'s pamphlet discusses the monitoring of a battery to insure proper charging, but emphasizes the necessity of "estimating" when a battery needs charging based upon its probable useful life (page 4) and the further requirement of charging with exact parameters.
According to Johnson Controls, Inc. overcharging a battery will result in a continued high current flow, which decomposes battery electrolyte water and ages the battery prematurely. Furthermore, undercharging a battery causes the current flow to stop before the battery is fully charged, which allows some of the lead sulfate to remain on the battery plates and prematurely ages the battery.
In response to these problems, Johnson Controls, Inc. recommends accurately estimating when a battery will need recharging and then properly charging it. It even discusses a simple solution to the problem by using Dynasty batteries of the same age, which allegedly minimizes adverse charging effects by limiting some of the guess work involved in estimating the failure of a battery.
In contrast to the necessity of "guess work" emphasized by Johnson Controls, Inc., Applicant's invention is significant because it provides a circuitry for predicting battery failure due to the normal aging process which as previously described results in slight changes of cell voltage while the battery is in a "float" stage.
Applicant's device is also signifigant because it is not primarily designed to measure possible failure of a battery while the battery is in a "load" mode that is, under specfic use. Previous attempts have been made to measure a battery's voltage while the battery is either charging or discharging during use. What Applicant's device does is rather to measure the potential failure of a battery when the battery is in a static, float mode in between uses.
Such previous attempts to measure a battery's voltage during charge or discharge have been disclosed in U.S. Pat. No. 3,546,576 of J. A. Frezzolini (Ogden Technology Labs, Inc.--assignee). It discloses a means for monitoring a battery during charge and discharge cycles to determine if the voltage remains within an acceptable range.
U.S. Pat. No. 4,020,414 of A. E. Paredes (Konar Corp.--assignee) discloses the use of a quad comparator chip with a scaling network and indicator lights to provide an indication of battery voltage during charge or discharge modes. Paredes specifically is used during a discharge mode referring to battery voltage as a gauge of capacity to forewarn the user of potential failure. This device cannot be used in a static, float mode such as Applicant's device. Paredes is designed to measure voltage across a battery either in an open circuit or in a loaded circuit with an analagous reference to available capacity remaining in the battery. The comparator used in the Paredes device is used as a driver for a digital voltmeter, whose voltage indication is displayed in a serial fashion via an array of light emitting diodes (leds).
U.S. Pat. No. 4,027,231 of D. R. Lohmann (U.S. Army--assignee) discloses a battery voltage indicator employing a scaling network with a plurality of transistor stages having LED indicators, each providing comparison against a Zener diode reference.
U.S. Pat. No. 4,028,616 of C. R. Stevens discloses employing a plurality of integrated op-amps with LEDs and a voltage dividing network to indicate battery voltage during charging or discharging use.
With regard to the Paredes patent, and actually any of the other references, there is a marked difference in operation and technique. Specifically, the Paredes device is designed to measure the voltage across a battery and indicate using either an open circuit or loaded circuit; the available capacity remaining in the battery. It does this by resolving small decreases along an analog slope that begins with a fully charged battery voltage. As that voltage decays it indicates with some relative accuracy the batteries available capacity to some predetermined end voltage. In other words there is a correlation between the open circuit or loaded circuit voltage of a battery and its available capacity.
The capacity in this case is stated in terms of a percentage. The comparator then in the Paredes patent is used as the driver for a digital voltmetering technique. The voltage comparator when combined with a resistor network is responsive in a digital sense to the increase or decrease of a DC voltage source and would function as a voltmeter device based on the value of the resistors used.
In contrast the Applicant's device uses the voltage comparator to indicate a voltage window. Even more important than that simple fact is that Applicant's device specifically is not used to determine battery capacity at all. Applicant's invention is used to predict failure of a battery. Herein lies the difference.
To further explain the difference that exist between Applicant's device and Paredes, it is necessary to explain what one is actually dealing with.
All the other inventions including Paredes deal with the battery primarily under load; that is to say in use as a voltage producing device. With Applicant's invention one is specifically making one's measurement only under the following specific conditions:
1. The battery is neither under load nor open circuit. PA1 2. The battery is in a float-charge condition. PA1 3. The battery charger is of the constant voltage/current limited type.
What Applicant is resolving is a shift in voltage across the battery due to a change in internal resistance. The change would be caused by either an internal shorting or opening of the battery itself. This effect will cause the voltage (IR) to either drop or increase slightly across that portion of the battery. This shift of voltage would not be perceived by measuring the entire battery and would only be perceived at the terminals of the offending cell/battery during the measurement conditions mentioned above.
Also, the unit when it is first set up, is adjusted to compensate for the batteries' internal resistance when newly installed, which can be different for each individual battery or cell in a given string. Therefore, in the final analysis, it is the specific voltage shift of a given battery cell that more precisely predicts imminent failure.
This is far different than using the gradual decline of cell voltage under load to perceive available battery capacity, as is being done in the Paredes patent. Additionally, the product applications are very different. In the Paredes example one is dealing with an automotive or marine type application, where batteries are not float charged on a continuous basis. The device Applicant designed is specifically used for stationary applications where the battery is float charged. While there are possibilities for its' use in "motive" applications, such as certain shipboard or aircraft applications, the measurement conditions Applicant's invention requires are generally not available in motive applications.